(a) Field of the Invention
The present invention relates to a method of preparing a flame retardant polyester fiber, and a flame retardant polyester fiber.
(b) Description of the Related Art
Polyester resins are generally available in many fields such as fibers, films, plastic moldings, and so on because of their superior mechanical strength, heat-resistance, and chemical resistance. However, the polyester resins have a disadvantage of rapid burning after being ignited, and many studies for granting flame retardancy to the resin are actively ongoing.
As a method for granting the flame retardancy to the polyester fiber, a method for coating flame retardants on the fiber surface after spinning and a method for blend-spinning of the polyester fiber wherein un-reactive flame retardants are added before spinning and after the polymerization thereof have been suggested.
As the flame retardants, compounds containing a halogen, mixtures of metal oxides, compounds containing nitrogen, compounds containing phosphorus, and the like have been utilized, and a method of drawing and heat-treating the fiber after spinning the spinning composition was used according to the general direct spinning drawing so as to promote the properties of the fiber.
However, in the case of coating the flame retardants on the fiber surface, there are some disadvantages such as incurring expense caused by the separate post-process for coating the flame retardants, the deterioration of the fiber properties caused by the mechanical abrasion during the post-process, and the deterioration of the efficacy as it is used repeatedly, because the flame retardants are merely coated on the fiber.
Furthermore, the blend spinning method of adding the non-reactive flame retardants before spinning and after polymerization thereof has some disadvantages of deterioration of the flame retardancy and the spinability of the fiber, when the non-reactive flame retardants are not uniformly mixed therein.
Furthermore, a method of making a polyester resin flame retardant by copolymerizing or blending polyester with the flame retardants during the polymerization process and preparing a polyester fiber from the resin was suggested. However, the method has a disadvantage that the properties of the fiber deteriorate according to the kinds of the flame retardants, although such method may improve the durability and the flame retardancy of the fiber in comparison with the prior method. For example, when the method uses a halogen-based flame retardant, the flame retardancy may be improved, but the polyester resin copolymerized with the flame retardant discolors and the light-resistance thereof deteriorates, and the use of flame retardant is limited because it is regarded as an environmentally harmful material. Furthermore, when the phosphorus-based flame retardant is used, there are some problems such as decreasing of thermal stability, discoloration, and deterioration of the quality of the final product.
To overcome these problems, Japan patent publication Sho 57-212251 and Sho 59-191716 suggested a method of preparing a flame retardant polyester fiber with a flame retardant polyester resin, wherein the flame retardant polyester resin is prepared by introducing a flame retardant and a polycondensation catalyst after the polycondensation is started and proceeded to some degree during the copolymerization of the polyester resin. But the method did not resolve the prior problems.
Also, when a fiber is prepared by using a general flame retardant copolymer, the drawing property and the thermal stability and so on deteriorate and it is difficult to obtain the properties required to the flame retardant polyester fiber, because the copolymer is in a heterogeneous state owing to the flame retardants copolymerized in the main chain of the resin.
Furthermore, when a flame retardant is copolymerized or blended with the polyester resin in order to obtain the flame retardancy, the maintaining time of the softening state during the solid state polymerization process becomes longer in comparison with the pure polyester resin because the melting temperature gets lower and the cold crystallization temperature gets higher than a pure polyester resin. Accordingly, there is a problem of a lumping phenomenon in which the polyester chips are coagulated in the solid state polymerization device owing to the high temperature during the solid state polymerization.
To resolve the problem, a method that slowly polymerizes the polyester while elevating the temperature stepwise from low to high during the solid state polymerization process was generally used. However, there are some problems that much powder is generated by the abrasion between the polyester chips and discoloration (dark yellowish) of the chips occurs as the time of the solid state polymerization increases, and consequently the productivity of the fiber is decreased owing to the reduction of the properties thereof.
Additionally, the flame retardant polyester fiber prepared by the prior method generally shows high thermal stress and has a disadvantage of low shape stability to heat.